Modern hard disc drives comprise one or more rigid discs that are coated with a magnetizable medium and mounted on the hub of a spindle motor for rotation at a constant high speed. Information is stored on the discs in a plurality of concentric circular tracks by an array of transducers ("heads") mounted to a controllably positionable actuator for radial movement relative to the discs.
Typically, such radial actuators employ a voice coil motor to position the heads with respect to the disc surfaces. The heads are mounted via flexures at the ends of a plurality of arms which project outward from an actuator body. The actuator body pivots about a cartridge bearing assembly mounted to the disc drive housing at a position closely adjacent the outer extreme of the discs so that the heads move in a plane parallel with the surfaces of the discs.
The voice coil motor includes a coil mounted radially outward from the cartridge bearing assembly, the coil being immersed in the magnetic field of a magnetic circuit of the voice coil motor. The magnetic circuit comprises one or more permanent magnets and magnetically permeable pole pieces. When current is passed through the coil, an electromagnetic field is established which interacts with the magnetic field of the magnetic circuit so that the coil moves in accordance with the well-known Lorentz relationship. As the coil moves, the actuator body pivots about the pivot shaft and the heads move across the disc surfaces.
A closed loop digital servo system such as disclosed in U.S. Pat. No. 5,262,907 issued Nov. 16, 1993 to Duffy et al., assigned to the assignee of the present invention, is typically utilized to maintain the position of the heads with respect to the tracks. Such a servo system obtains head position information from servo blocks written to the tracks during disc drive manufacturing to maintain a selected head over an associated track during a track following mode of operation. A seek mode of operation, which comprises the initial acceleration of a head away from an initial track and the subsequent deceleration of the head towards a destination track, is also controlled by the servo system. Such seek operations are typically velocity controlled, in that the velocity of the head is repetitively measured and compared to a velocity profile, with the current applied to the coil being generally proportional to the difference between the actual and profile velocities as the head is moved toward the destination track.
A continuing trend in the industry is to provide disc drives with ever increasing data storage and transfer capabilities, which in turn has led to efforts to minimize the overall time required to perform a disc drive seek operation. A typical seek operation includes an initial overhead time during which the disc drive services its own internal operations, a seek time during which the head is moved to and settled on the destination track, and a latency time during which the drive waits until a particular sector on the destination track reaches the head as the discs rotate relative to the heads.
Seek times have typically been minimized through the application of relatively large amounts of current to the coil during the acceleration and deceleration phases of a seek operation. One way of reducing seek time is to increase the relative amount of current to the electric coil. However, as the current is increased the operating temperature of the coil likewise increases, as a proportionate amount of the electrical energy is dissipated as heat energy. One skilled in the art will understand that the amount of current that can be passed through a coil is generally a function of its electrical resistance, which is directly affected by the temperature of the coil. As the temperature of the coil increases, the resistance of the coil increases, and the magnitude of the control current is limited, thereby adversely affecting the drive seek time. Moreover, elevated coil temperatures can also adversely affect the seek time performance by generally weakening the strength of the magnetic circuit of the magnet assembly.
Additionally, elevated voice coil motor temperatures can result in the degradation of adhesive and insulative materials used in the construction of the voice coil motor. Such degradation can lead to internal contamination of the disc drive as well as to the shorting of the coil.
Efforts have been made to reduce such temperature increases by using external means to cool the voice coil motor. For example, U.S. Pat. No. 5,517,372 issued May 14, 1996 to Shibuya et al., discloses a means for diverting the air flowing over the discs to flow over the voice coil motor. However, such cooling efforts increase power consumption by creating increased drag upon the discs. Such methods in essence add to the complexity of the drive through the addition of extraneous items such as ducts or diverters.
There is a continuing need in the industry for an improved actuator assembly with enhanced heat dissipation to facilitate cooling of the actuator coil without hindering the overall performance of the disc drive.